A lot of episodes that seem like filler when you first watch them become important in retrospect. The Dominion were introduced in a seemingly-inconsequential Ferengi episode.
Personally I consider Rocks and Shoals
too important to be a filler episode. Seeing Kira WAKE UP was a huge part of the story. Just because an episode can be skipped over without losing too much of the story doesn't make it filler.
No one would call The Inner Light
or City on the Edge of Forever